1. Field of the Invention
The present invention relates to a developing apparatus to be used in an image forming apparatus using an electrophotographic method or an electrostatic recording method such as a copying machine, a printer, or a facsimile.
2. Related Background Art
In an image forming apparatus using an electrophotographic method, especially an image forming apparatus forming an image having a chromatic color, a two-component development method involving the use of a mixture of non-magnetic toner (toner) and a magnetic carrier (carrier) as a developer has been conventionally widely used. The two-component development method has advantages over other development methods currently proposed such as the stability of image quality and the durability of an apparatus.
In a developing apparatus using the two-component development method (two-component developing apparatus), when toner is supplied to an image bearing member to perform development, the toner concentration in a developer (a ratio between toner and a carrier, or a ratio of toner to the entire developer) gradually reduces. New toner must be replenished in order to prevent the reduction in toner concentration. A two-component developer containing a carrier is supplied in some cases to replenish toner.
FIG. 7 shows a conventional general two-component developing apparatus. An example of the prior art document relating to such two-component developing apparatus includes Japanese Patent Application Laid-Open No. S55-32060.
A developing apparatus 101 has a developer container 102 for storing a developer. A portion of the developer container 102 opposed to an image bearing member as a member to be developer is opened. A developing sleeve 111 as a developer carrying member for supplying a developer to the image bearing member is rotatably disposed so as to be partially exposed from the opening. The developer container 102 has a developing chamber (a first developer storing chamber) 121 and an agitating chamber (a second developer storing chamber) 122. The developing chamber 121 is disposed along an axial direction of the developing sleeve 111, and the developer stored in the chamber is supplied to the developing sleeve 111. In the agitating chamber 122, the replenished toner and the developer in the developer container 102 are carried in a direction opposite to the developing chamber 121 while they are mixed and agitated. The developing chamber 121 and the agitating chamber 122 are partitioned by a partition wall 125. However, a first connection portion 123 and a second connection portion 124 permitting the passage of a developer are formed on both longitudinal ends of the partition wall 125. In addition, a first screw 113 and a second screw 114 are arranged as carrying members for carrying and agitating a developer (developer carrying and agitating members) in the developing chamber 121 and the agitating chamber 122, respectively. With this configuration, a developer circulating path is formed, in which a developer is circulated between the developing chamber 121 and the agitating chamber 122 through the first connection portion 123 and the second connection portion 124 (a direction indicated by an arrow D in the figure).
The developer container 102 is provided with a toner replenishment port for replenishing toner. At a toner replenishment position T, toner replenished to the developer container 102 as a result of falling by virtue of the force of gravity is agitated with the developer in the developer container 102 in the agitating chamber 122. Thus, the replenished toner contacts with a carrier to be frictionally charged.
However, when agitation in the agitating chamber 122 is insufficient, the developer is carried to the developing chamber 121 through the first connection portion 123 while the replenished toner is not sufficiently charged. Furthermore, when the insufficiently charged toner is used for a development operation, toner fog occurs on an image white portion (a region on an image bearing member to which toner should not adhere originally), so a reduction in image quality may occur.
To prevent such toner fog, it is sufficient that the replenished toner be sufficiently charged during carriage to the first connection portion 123. In the conventional developing apparatus 101, a distance L from the toner replenishment position T to the first connection portion 123 in the agitating chamber 122 (hereinafter, referred to as an “agitation distance”) is appropriately set for sufficiently charging the replenished toner.
However, in the conventional developing apparatus 101, as a result of appropriately setting the agitation distance L for sufficiently frictionally charging toner replenished to the developer container 102, the agitation distance L has often been longer than the width of an electrostatic image to be formed on the image bearing member in a direction toward the longitudinal direction of the developer circulating path, that is, a development region (hereinafter, referred to as an “image formation width”) G. In general, the image formation width G substantially corresponds to the length of a developing sleeve 211 in an axial direction (longitudinal direction).
For example, in the developing apparatus 101 shown in FIG. 7, the toner replenishment position T is placed on an upstream side of a developer carrying direction with respect to the image formation width G, whereby the agitation distance L necessary for sufficiently charging replenished toner is secured.
In general, the minimum length in the longitudinal direction requested for the developing apparatus 101 is very the image formation width G. However, in the developing apparatus 101 shown in FIG. 7, the agitation length L is longer than the image formation width G in order to sufficiently charge replenished toner during carriage of the toner to the first connection portion 123. As a result, a reduction in size of the developing apparatus 101 is inhibited, and a reduction in size of the entire image forming apparatus is inhibited.
With recent increasing needs for images having chromatic colors, not only a reduction in cost of an image forming apparatus forming an image having a chromatic color but also a reduction in size of the apparatus has been demanded as in the case of, for example, a monochromatic image forming apparatus according to an electrophotographic method.
An idea of improving an ability to charge replenished toner without any increase in the dimension of the developing apparatus 101 in the longitudinal direction as described above is, for example, as follows.
The carrying member 114 is obliquely disposed, a position on an upstream side of the developer carrying direction is lowered, and a position on a downstream side thereof is raised, so part of a developer back-flows owing to the force of gravity. Thus, the developer that has back-flown falls from above toner replenished to the developer container 102 and floating on the developer surface, whereby an ability to take the toner in the developer is improved. As a result, the replenished toner is quickly mixed with the developer, whereby an ability to charge toner can be improved.
However, under a more stringent condition, for example, when the replenished toner amount is large, when a large replenished toner amount continues for a long period of time, or when a developer deteriorates as a result of long-term use, a sufficient agitating ability is not obtained in some cases if the carrying member is merely obliquely disposed while the behavior of the developer surface in the developer container is not taken into consideration. Accordingly, an additional improvement of an ability to agitate a developer has been demanded.
The invention described in Japanese Patent Application Laid-Open No. 2003-5519 discloses a configuration in which a second screw 227 is arranged obliquely with respect to a first screw 226 as shown in FIGS. 8 and 9. In addition, in the developing apparatus described in Japanese Patent Application Laid-Open No. 2003-5519, a developer falling space 234 in which a developer carried by the first screw 226 falls into the second screw 227 is formed between an end portion of the first screw 226 on a downstream side of a developer carrying direction and an end portion of the second screw 227 on an upstream side of the developer carrying direction. In addition, an inclined plane 235 for guiding the developer to the second screw 227 is arranged below the developer falling space 234. The developer and replenished toner fall onto the inclined plane 235 through the developer falling space 234. Then, the developer and the replenished toner are overlaid and agitated on the inclined plane 235, slide along the inclined plane 235, and are carried to the end portion of the second screw 227 on the upstream side of the developer carrying direction. With this configuration, an improvement of, for example, the dispersibility of the replenished toner into the developer has been attempted. However, in the developing apparatus described in Japanese Patent Application Laid-Open No. 2003-5519, the second screw 227 is merely slanted with respect to the first screw 226 in order to secure the developer falling space 234. The developing apparatus described in Japanese Patent Application Laid-Open No. 2003-5519 has neither disclosure nor suggestion regarding an improvement of an ability of the second screw 227 to agitate a developer achieved by an appropriate back-flow of the developer carried by the second screw 227.